AQA P3 Topic 1 - The Polesworth School

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Transcript AQA P3 Topic 1 - The Polesworth School

AQA P3 Topic 1
Medical applications of Physics
X rays
X-ray properties
• X-rays are part of the electromagnetic spectrum.
• X-rays have a wavelength of the same order of magnitude as the diameter of an atom.
• X-rays are ionising.
• Different materials absorb different amounts of x-rays
• Denser material = more absorption = looks lighter on the x-ray photo
• X-rays use charged-couple devices (CCDs) to produce an image
CT Scans
• X-ray source moves in circle around patient
• Detectors opposite the source
• Many cross-sectional images that can build up 3D image
• Tumours detected with areas of brightness or dark patches
 Painless and non invasive
 Can eliminate the need for biopsy to decide on treatment.
 Can be used to destroy tumours at or near the surface of the body.
 Both give a dose of radiation equivalent to 10 yrs background radiation
 Increased risk of cancer so not recommended on children or pregnant females.
• Humans detect sound waves between 20 – 20,000Hz
• Above 20,000Hz = Ultrasound
• Some animals use it to communicate (dolphins)
Ultrasound Scan
• Make images of things inside the body
• Ultrasound waves are partially reflected when they meet a
boundary between two different media (types of body tissue).
• Ultrasound is non-ionising so it is safer than an X-ray for use in
Distance measurements
• The distance between interfaces in various media can be
calculated using;
s = v x t where s= distance in metres, m
v= speed in meters per second, m/s
t = time in seconds, s
Use in medicine
• Pre-natal scanning and the removal of kidney stones.
Refractive index
• Is the change of direction of light as it
passes from one medium to another.
Refractive index
The refractive index of a material, n, is a
measure of how much a substance can refract
a light ray.
Angle of incidence, i
Angle of refraction,
actual location
Other applications using light
Optical Fibre
Light ray is consistently totally internally reflected as it
is at an angle greater than the critical one.
Endoscope – look inside a patient.
 Flexible rod of optical fibres.
 Used to see inside the body directly.
Critical angle
The critical angle is the angle of incidence of a light ray in a transparent
substance that produces refraction along the boundary.
Power of a lens =
(dioptre, D)
Focal length(metre, m)
• Lenses are optical devices that bend light by refraction:
• In a converging (convex) lens, the light rays come together.
• In a diverging (concave) lens, the light rays spread apart.
A real image can be projected onto a screen. A virtual image cannot
be projected onto a screen. Magnified means made bigger,
while diminished means made smaller. Inverted means turned upside
Converging Lenses
 Converging lens – parallel rays refracted
and meet at focal point
 Lens to focal point = focal length
The focal length is the distance between
the focus and the lens axis. This is given the
code F. So 2F means twice the focal length.
Object at
Image at
Image on a film
(at infinity)
Between F and
Image on a film
(close up)
Same size
Spot light
Magnifying glass
Between 2F and
<F (on same
Diverging Lenses
 Diverging lens – focal point is point rays
seem to coming from
 Focal point to lens = focal length
Ray Diagrams for Converging Lens
Draw the principal axis and the lens (always shown as a line with arrowheads)
Draw the object (drawn as a vertical arrow going upwards )
Draw a line straight through the principal axis and the lens line (Does not
Draw a line parallel to the principal axis that refracts at the lens and goes
through the focal point
Draw the object on the other side of the lens where both lines cross.
Ray Diagrams for Diverging lens
Draw the principal axis and the lens (always shown as a line with arrowheads)
Draw the object (drawn as a vertical arrow going upwards )
Draw a line straight through the principal axis and the lens line (Does not
Draw a line parallel to the principal axis that refracts as if it came from the focal
point on the same side as the object.
Draw a dotted line (this is a virtual ray) to show where the ray appears to come
Draw the object where the virtual ray crosses the first ray. (Image will be virtual)
The magnification of a lens is the ratio of the size of the image to the size of the
object (how many times bigger the image is than the object was!). If the ratio is
greater than 1 the image is bigger than the object (enlarged) and if it is less than 1 it
is smaller than the object (diminished in size).
Since v is in metres,
and u is in metres, M
has no units
The Eye
Image formation
• Light converges on the retina
• Path of rays is changed by the eye by refraction (carried out by cornea and lens)
• Ciliary muscles change the shape of the lens to keep image focussed on retina if the distance alters.
– Contracted ciliary muscles = loose ligament = lens more rounded = focus on nearby objects
– Relaxed ciliary muscles = taut ligaments = lens flattened = focus on distant objects
• No limit to how far away you can focus – far point is at infinity
• You near point is approx. 25cms – nearer and image is blurred.
Accommodation/ Focussing
Eye structure diagram
The Eye
In a normal eye, the lens focuses the
image exactly on the retina.
However, in the eye of a person with
Myopia (short sighted), the image is
formed before the retina. This leads to a
blurred image.
Short sight can be corrected by glasses that have a
concave (diverging) lens. This causes the light rays to
diffract outwards slightly as they pass the lens so that
they are focused exactly on the retina by the lens in
the eye.
The Eye
In a person with ‘hyperopia’ (long sight),
the image is not correctly focused onto
the retina by the eye lens. The image is
focused behind the retina, leading to a
blurry image.
Long sight can be corrected by using a
convex (converging) lens. This causes the
light rays to converge slightly before they hit
the lens so that they are refracted perfectly
onto the retina.
The Eye
AQA P3 Topic 2
Using Physics to make things work
Centre of Mass
The centre of mass of an object is that
point at which the mass may thought to
be concentrated.
The centre of mass of
symmetrical objects
ALWAYS lies along the line
of symmetry of the object.
If you suspend an object and then release it, it will soon
come to a rest. When this happens, the centre of mass
will be directly below the point of suspension. The
object can be said to be in equilibrium.
The pendulum moves backwards and forwards
and always returns back to the middle, called
the equilibrium position. This type of motion
is called oscillating motion.
The time period of a pendulum is the time it
takes for a pendulum to complete one full
cycle of motion. The easiest way to measure
this is the time it takes for the pendulum to
swing from one side of the pendulum to the
other side and back again.
The factors that affect the time period of a
The time period of a pendulum can be
pendulum are:
calculated using the following formula:
1. The length of the pendulum
2. The amplitude (maximum displacement)
of the swing
T = Time (s)
f = frequency (Hz)
The turning effect of a force is called a moment
Moment = force (N) x distance from pivot (m)
A seesaw is an example of the principle of
moments. This states that for an object in
equilibrium (not moving!) the sum of all
the clockwise moments about any point is
equal to the sum of all the anticlockwise
moments about the same point.
Clockwise moments = anticlockwise
W1 x D1 = W2 x D2
The weight of an object acts through the
centre of mass. As the object is initially
tilted, the weight is causing an
anticlockwise moment about the pivot. If
the object is let go, the moment will cause
the object to go back onto its base.
When the object has been tilted beyond a
certain point, the weight will now cause a
clockwise moment about the pivot. If the
object is let go, the moment will cause the
object to topple over!
If the line of action of the weight of an
object lies outside the base of the object
there will be a resultant moment and the
body will tend to topple.
Pressure = pascals (Pa)
Force = Newtons (N)
•Area = metres² (m²)
Liquids are almost incompressible. This means that if a force is applied
to liquid in one part of the system, it will move and transfer the force to
another part of the system.
Circular Motion
When an object moves in
a circle it continuously
accelerates towards the
centre of the circle.
This acceleration changes
the direction of motion of
the body, not its speed.
The resultant force
causing this
acceleration is called
the centripetal force
and is always directed
towards the centre of
the circle.
The centripetal force needed to make an object perform circular
motion increases as:
•The mass of the object increases.
•The speed of the object increases.
•The radius of the circle decreases.
AQA P3 Topic 3
Keeping things moving
The Motor Effect
Electric currents always produce a magnetic
field. The magnetic field of a single current
carrying wire is like this:
If we put a current carrying wire in a
magnetic field, we see that there is a
Direction (movement) of the force can be found
using Fleming’s left hand rule.
The size of the force can be increased
 Increasing the strength of the
magnetic field
 Increasing the size of the current
Electromagnetic induction
P1.26 Generating Electricity
A Generator
• Current is induced in the
coil. And transferred to a
circuit through the slip
rings which touch carbon
• As the coil turns the
direction of the induced
current changes.
• Alternating Current (A.C.)
is induced
P1.26 Generating Electricity
A Dynamo
• Used to produce
electricity to power cycle
• Magnet spins inside a coil
of wire
• A current is induced.
• Direct current (D.C)
• Other examples are wind
up torches or radios.
P1.27 Transmitting Electricity
Key words:
• Transformers – change the size of an alternating
• Step up transformer – increases the voltage
• Step down transformer – decreases the voltage
Electricity is transmitted at high voltages as it
increases efficiency by reducing heat loss in the
power lines.
P1.27 Transmitting Electricity
• A transformer consists
of two coils wrapped
around an iron core.
• Electricity is supplied
to the primary coil
and obtained in the
secondary coil at a
different voltage.
You need to be able to use this formula to work out the turns of
the voltage:
Try these examples…
P1.27 Transmitting Electricity
Voltage increased to
400,000V to reduce
heat loss
Voltage reduced to
33,000V for factories
Hazards of electrical transmission
• High voltages are likely to kill.
Voltage reduced to
230/240V for homes